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Labour's new deputy leader Kelvin Davis has shown a willingness to be his party's attack dog - calling the Health Minister the "doctor of death" and comparing Bill English's personality to that of a rock.

Davis appeared along with new Labour leader Jacinda Ardern on TVNZ's Q+A programme this morning.

Ardern again said she wanted her campaign to reflect "relentless optimism", but Davis was happy to take some pot shots at National.

"Everything has changed. The vibe has changed. The momentum has changed. The energy has changed. The election has changed," Davis, the MP for Te Tai Tokerau, said.

"We all know about the Jacinda effect - pure optimism. When we look over at the other side there and we see a Prime Minister with the personality of a rock.

"We have got Jonathan Coleman the doctor of death, we have got Steven Joyce who is as authentic as a $4 Rolex, we've got Gerry Brownlee who has got the energy of a small hill, we've got Simon Bridges - the only person under 80 who still buys Brylcreem, and Judith Collins, look her stare caused that ice shelf in Antarctica to crack off and float away."